Friday 9 September 2011 13.25 EDT
First published on Friday 9 September 2011 13.25 EDT

Vehicle checkpoints were in place in key locations across New York on Friday, with police stopping vans and lorries passing through the city in response to specific and credible intelligence that a car bomb was planned to disrupt the 10th anniversary of 9/11.

The Guardian understands the dramatic increase in security was carried out on the basis of an alert that came from a long-term intelligence source who has proved reliable in the past. But the source picked up the information secondhand, which is why it is being portrayed as "reliable but unconfirmed".

On Thursday night Hillary Clinton said the plot originated with al-Qaida and its target was New York and Washington.

The department of homeland security has not raised the terrorist alert level, but this is being considered.

As the final touches were being put in place for Sunday's commemoration of the anniversary at Ground Zero, which both President Obama and his predecessor, George Bush, are expected to attend, the heightened security that had been promised by several federal and local agencies was clearly visible on the streets of Manhattan.

All lorries were being stopped on George Washington Bridge and there was increased security at all other bridges and tunnels.

A roadblock was set up in the middle of Times Square itself, and on main cross-streets leading into it, causing virtual gridlock. Despite the inconvenience, truck drivers who were the focus of police attention showed no signs of disgruntlement.

"I'm happy in a way, as the truck in front of me could be stuffed with explosives," said Henry Simon, shortly after his removal van was stopped and searched.

A Scottish choir of 123 singers formed by the Lothian & Borders and Tayside police choirs went ahead with an impromptu performance in Times Square, but cancelled its plans to sing on the subway, fearing that they might be suspected as a decoy. "We are here to show Scotland's compassion for those who lost their lives on 9/11, we don't want to upset anyone," said the musical director Andrew Russel.

As New York knuckled down for what promises to be a tense weekend, US intelligence agencies were continuing to try to track down three men who, according to the threat warning, had entered the US since August with the aim of launching an attack.

The report of a plot, which the president was told about on Wednesday night, was said to be plausible and more specific than usual, although it came from one source and was uncorroborated.

Clinton said the threat had been made public so as to activate a "great network of unity and support" against those who would wreak violence on innocent people. "It is a continuing reminder of the stakes in our struggle against extremism," she said.

The vice-president, Joe Biden, said the intelligence gathered by the CIA suggested the three men might be planning a vehicle-based attack. "We do have talk about using a car bomb. But we do not have confirmation of that – we don't have a smoking gun," Biden told ABC News.

The CIA is working with Pakistan to gather information about the men, who may be from the Pakistan-Afghanistan border region. One of the men is reported to be a US citizen, and the other two may have had US papers, and intelligence officials are understood to be scouring databases to track several names of individuals who entered the country after mid-August.

Congressman Pete King, who sits on the House intelligence committee and receives regular briefings from the agencies, told CNN the level of detail stretched to how it would be carried out and that "it is plausible, though there are some holes in it".

A US official said: "US counter-terrorism entities have been alert for any leads, and for the first time we've received specific and credible – but unconfirmed – threat information linked to the 9/11 anniversary. Any and all leads related to possible plots are of course being run to ground."

One of the key findings of the 9/11 commission report that looked at the events leading up to the attacks on New York and Washington 10 years ago was that there were ample warnings in the weeks leading up to 9/11 of a massive attack in the pipeline, yet the intelligence was not acted upon and shared between agencies.

Everybody involved in the current scare, from Obama and the CIA through to the FBI and NYPD, is very aware of that finding and determined to avoid making the same mistakes again.